Cellular communications systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. Analog cellular systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular systems such as designated IS-54B and IS-136 in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
Frequency reuse is commonly employed in cellular technology wherein groups of frequencies are allocated for use in regions of limited geographic coverage known as cells. Cells containing equivalent groups of frequencies are geographically separated to allow mobile units in different cells to simultaneously use the same frequency without interfering with each other. By so doing many thousands of subscribers may be served by a system of only several hundred frequencies.
In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands called channels. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 MHz. At present there are 832, 30-KHz wide, radio channels allocated to cellular mobile communications in the United States. To address the capacity limitations of this analog system a digital transmission standard has been provided, designated IS-54B, wherein these frequency channels are further subdivided into time slots. The division of a frequency into a plurality of time slots wherein a channel is defined by a frequency and a time slot is known as time division multiple access (TDMA).
As illustrated in FIG. 1, a cellular communication system 20 as in the prior art includes one or more mobile stations or units 21, one or more base stations 23 and a mobile telephone switching office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may comprise hundreds of base stations, thousands of mobile stations and more than one MTSO. Each cell will have allocated to it one or more dedicated control channels and one or more voice channels. A typical cell may have, for example, one control channel, and 21 voice/data, or traffic, channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information.
The MTSO 25 is the central coordinating element of the overall cellular network 20. It typically includes a cellular processor 28, a cellular switch 29 and also provides the interface to the public switched telephone network (PSTN) 30. Through the cellular network 20, a duplex radio communication link 32 may be effected between two mobile stations 21 or, between a mobile station 21 and a landline telephone user 33. The function of the base station 23 is commonly to handle the radio communication with the mobile station 21. In this capacity, the base station 23 functions chiefly as a relay station for data and voice signals. The base station 23 also supervises the quality of the link 32 and monitors the received signal strength from the mobile station 21.
In a mobile communications system, signal performance my be reduced due to signal fading occurring as a result of physical interference and motion of the mobile user terminal. Fading can be reduced, for example, by increasing transmitter power, antenna size, and antenna height. These solutions, however, may be impractical and/or costly.
Accordingly, multiple transmit antennas have been used to provide transmission diversity as discussed for example in the reference by Guey et al. entitled "Signal Design for Transmission Diversity Wireless Communication Systems Over Rayleigh Fading Channels." (Proceedings IEEE VTC, 1996). The disclosure of this reference is hereby incorporated herein in its entirety by reference. If the antennas are placed far apart, each signal will experience independent fading. This diversity can be made accessible to the receiver by switching between the transmitters at different time instants. The peak to average power ratio of the transmitted signal may be greatly increased, however, and the output amplifier design may be complicated.
Other transmission diversity techniques that do not switch between transmitters are ones using an intentional time offset or frequency offset, phase sweeping, frequency hopping, and/or modulation diversity. Most of these techniques use phase or frequency modulation of each transmitter carrier to induce intentional time-varying fading at the receiver. In addition, coded modulation schemes have been proposed to access the diversity of a multiple transmitter system without using an interleaver.
Notwithstanding the transmission diversity techniques discussed above, there continues to exist a need in the art for improved diversity methods, systems, and terminals.